Imagine flying deep within the asteroid belt to study the most unreachable location in the solar system: the deep core of a terrestrial world.
That will be one asteroid mission that will be proposed for NASA’s upcoming competition to select its next Discovery mission to explore the solar system.
Asteroids are found scattered across the solar system like artifacts strewn across an archaeological site. Just as a delicate gold necklace and simple rough potsherd can speak the different strata of an ancient society, the many types of asteroids speak of the strata of conditions in the earliest eon of the solar system. Stony bodies, for example, formed closer to the sun while icy bodies formed further away.
By studying asteroids (and their cousins, comets), scientists can study the remains of conditions from the earliest solar system.
Most asteroids are smallish affairs with diameters measured in meters to kilometers or at most a few tens of kilometers and are usually chips knocked from larger protoplanets by impacts from still other asteroids. In a few cases, though, the original protoplanets remain largely intact. By studying these worlds with spacecraft, geologists can examine how the terrestrial planets formed. On the true planets, that early history is long lost because of geologic activity.
Some of the protoplanets have a familiar structure, like Vesta with its rocky mantle and metallic core that resembles the structure of the terrestrial planets. Others are unlike any world we have explored to date. Massive Ceres is a rock-ice world with a deep mantle of ice and is a member of a family of asteroids that emit water vapor (some are even comet-like with full dust and ice vapor trails). The asteroid Psyche is a metal world that may be the remnant core of protoplanet.
(When I first began reading about the solar system several decades ago, comets were icy balls with some dust and asteroids were rocky. Now we know that comets and asteroids are a continuum and many asteroids contain substantial amounts of ice that would have been water when these bodies still retained the heat form their formation.)
Asteroids have been popular targets for solar system missions. NASA has flown two asteroid missions and is building a third with a fourth listed as high priority. Japan’s JAXA space agency has flown one mission, is building a second, and is planning for a third. American and European scientists have proposed numerous additional missions. In the last competition for NASA’s low-cost Discovery program (~$425M missions), over a quarter of the 28 missions proposed would have flown to an asteroid or observed them with a space telescope.
In the competition for the next Discovery mission expected to begin this year, we can expect a similar enthusiasm from the scientific community. A good portion of the proposals will probably be to fly to one or more of the small asteroids whose orbits are near Earth’s. Flights to these worlds are relatively easy, making the missions that orbit them and even land budget bargains and low risk. In addition, collisions, gravitational influences of the true planets (especially Jupiter), and even the pressure of the Sun’s light have scattered the smaller asteroids across the solar system. Within easy reach from Earth are bodies representing a plethora of conditions that were found across the early solar system.
Scientists who want to study protoplanets have to look to the asteroid belt between Mars and Jupiter. There the Discovery Dawn spacecraft has finished its studies of the rocky protoplanet Vesta and is headed towards the rock-ice dwarf planet Ceres.
While the Dawn mission picked off two of the most exciting protoplanets, there are plenty more. European scientists in their last mission competition proposed two main belt asteroid missions (neither selected). They focused on two classes of asteroids. The first were asteroids that have been observed to eject jets of water vapor. The second class was actually a single asteroid – the metal world Psyche.
Scientists planning to propose Discovery missions usually are reluctant to say much about their ideas. The competition is tough and missions usually are proposed several times. Why say something that would give a competing team a good idea? So of the eight asteroid Discovery missions proposed last time, we know very little about what was actually proposed.
Even with the stiff competition though, some scientists try to build support for their proposals by making some disclosures in public. It can’t hurt to have the members of the review panels already excited by a mission before they evaluate the proposal. In general, we hear more about how scientifically interesting a destination is (those facts are widely known) and less about the spacecraft and instruments (what I know many of the readers of this blog are most interested in). The few times we hear about detailed implementation, it generally is to a destination that seems beyond the reach of a Discovery-class mission where building technical credibility before the review likely helps. All three of the teams that have proposed missions to the Saturn system, for example, have revealed a fair amount about the implementation.
The team preparing to propose a mission to the metal world Psyche has been relatively open about their proposal although they talk more about the destination than their spacecraft and instruments.
Fortunately, the destination is worth learning about. Stop for a second and ask yourself – what location for all the planets do we know least about?
It’s the cores of the planets. We can infer much about the size of the cores from seismic data (so far available only for the Earth and the moon but soon also for Mars), gravitational studies that reveal the mass of the core, and measurements of the magnetic field when present. No one, though, has ever seen a planetary core or directly measured its composition.
The asteroid Psyche may give us that opportunity. As bodies coalesced in the early solar system, initially random chance brought dust and ice to clump together. Once a body reach a kilometer or more in diameter (what is called a planetesimal), its gravity became strong enough to pull more material in to it. At a critical size, the heat from radioactive elements, collisions, and gravitational pressure melted the interiors of these worlds and they became protoplanets. Iron and nickel metals sank to the cores, mantles formed from the lighter silicate materials or ices, and a crust may have formed of either unmelted primitive materials, or by volcanism from the interior flooding the surface.
Three possible origins have been suggested for the metallic asteroid Psyche (~250 km diameter), all of them intriguing.
Psyche could be an asteroid in which repeated collisions chipped off the crust and mantle, leaving the core a naked body. If this is the case, then a mission to this world would be the equivalent to a mission deep below the surface of any of the terrestrial planets to examine their cores. It’s a journey that is possible only because chance created and then preserved from ultimate destruction by further collisions Psyche’s naked core.
Psyche could be the remnant of the collision of two protoplanets that shattered and expelled the core of the smaller body to become Psyche. In this case, we wouldn’t get to examine an intact protoplanet’s core. We’d still get to examine the composition of a protoplanet’s core, though, and also see how a world composed almost purely of metal formed itself following a collision. Collisions such as this would be been common in the early solar system. One is believed to have created the Earth-moon system. All planetary cores, in fact, almost certainly formed from multiple generations of fragmentation, differentiation, and merging of previous cores.
And finally, Psyche could have formed so close to the early sun that all materials other than metals (and some silicates) would have been evaporated and have been unavailable for planet building. Later migrations of Jupiter and Saturn in and out of the inner solar system could have moved this world to its present location in the asteroid belt. In this case, a mission to Psyche would show us an entirely new class of world.
Telescopic observations reveal that Psyche’s surface is 90% metallic and 10% silicate rock. A spacecraft orbiting Psyche likely could distinguish between these scenarios by measuring the composition in detail and looking at the arrangement of the silicate material. If the silicate material is primarily high-magnesian pyroxene or olivine, then these silicates are likely the remnants of a crystallizing magma ocean, and indicate that Psyche started as a differentiated planetesimal and had its mantle stripped, validating the mission’s prime hypothesis for this body. If the silicates are all primitive chondritic material, then they were likely added as later impacts, and Psyche may have started life as a highly reduced metallic body without a significant silicate mantle, or, the nature of impact flux and its consequences are far more significant than our current models indicate. The numbers and shapes of craters on Psyche’s surface may help decipher that story.
Here are some of the key questions a spacecraft would explore at Psyche: How did this large metal world form? If it is a remnant core, what is the composition and structure of a terrestrial world’s core? If Psyche was once molten, did it solidify from the inside out, or the outside in? We have a number of meteorites from a single metallic asteroid (the group IVA iron meteorites); is Psyche their source? The metallic asteroids appear to be much less dense than the metallic meteorites; are these asteroids rubble piles?
We know a great deal about the geology and surface features of rocky worlds, icy worlds, and will learn about rock-ice worlds when the Dawn spacecraft reaches Ceres next year. Metal worlds may differ significantly in their appearance. What does the surface of a metal world look like? Imagine how strange a crater might appear. Laboratory tests of craters in metal show that sometimes the ejecta flaps freeze before they fall; could this happen on a planetesimal?
Like the Dawn spacecraft, the Psyche spacecraft would use solar electric propulsion. As for instruments, the proposal team’s Principal Investigator, Dr. Lindy T. Elkins-Tanton of the Carnegie Institution for Science, told me, “We hope to learn not just about the surface of this metal body, but also about its interior, which requires geophysics. We'll be carrying a magnetometer, and we'll use the spacecraft itself to develop a detailed model of the body's gravity field. With these measurements and knowledge of topography, we'll get information on internal structure.
“We'll have an imager, of course, in the hopes of seeing some unforeseen new metal geology, and to count craters to measure the age of the surface, among other goals. Measuring the surface compositions of a metal object remotely is more difficult. Infrared spectrometers are great for silicates, but only gamma ray spectrometers can measure metal composition. We expect, though, to see some silicate materials along with the metals.”
I asked Dr. Elkins-Tanton about the possibility of flying to a second asteroid. She told me that they looked at this and concluded that it wasn’t feasible. No other asteroid visit would address their questions about metallic asteroids (and the instrument suite that would be wanted for a comet-like asteroid, for example, would be different). It also would be difficult to fit second asteroid visit into a Discovery mission budget.
In my previous post, I wrote that Discovery mission competitions surprise and delight us with the cleverness of the missions proposed. While we will hear little about many of the missions likely to be proposed for the asteroids, the Psyche mission gives an idea of what is possible.
You can read a two page summary of the science goals for the Psyche mission at this link.
My thanks to Dr. Lindy Elkins-Tanton for reading a draft of this post and making several useful suggestions.